1. Field of the Invention
The present invention generally relates to an apparatus and method for transmitting/receiving data in a communication system. More particularly, the present invention relates to an apparatus and method for transmitting/receiving data using an Incremental Redundancy (IR) scheme.
2. Description of the Related Art
The provisioning of services with different Quality of Service (QoS) levels at high rates to users is an active research area for future-generation communication systems. Major future-generation communication systems are Institute of Electrical and Electronics Engineers (IEEE) 802.16d and IEEE 802.16e based systems. IEEE 802.16d differs from IEEE 802.16e in whether mobility is supported.
FIG. 1 illustrates a cell configuration for a typical IEEE 802.16e communication system.
Referring to FIG. 1, a single cell includes a Base Station (BS) 110 and a plurality of Mobile Stations (MSs) 120 to 150 (MS A to MS D). MS A to MS D are spaced from the BS 110 by different distances. In general, MS A to MS D are placed in different channel statuses according to the distances. The nearest MS, MS A is in the best channel status and the remotest MS, MS D, is in the most inferior channel status. The channel statuses between the MSs 120 to 150 and the BS 110 are affected by obstacles in between or interference from other signals as well as the distances. The channel statuses are estimated taking into account the distances only, by way of example.
Because of the different channel statuses of the MSs 120 to 150 within the cell, the BS 120 selects Modulation and Coding Scheme (MCS) levels for the MSs 120 to 150 according to their channel statuses and sends signals to the MSs 120 to 150 at the MCS levels. However, when sending common data to the MSs 120 to 150, the BS 110 uses the most robust of the available MCS levels so that the common data can be received normally even at the MS in the most inferior channel status, i.e. MS D. For notational simplicity, Multicast/Broadcast Service (MBS) data is taken as an example of the common data.
FIG. 2 illustrates transmission and reception of MBS data in the typical IEEE 802.16e communication system.
Referring to FIG. 2, upon the generation of MBS data 211 to be sent, the BS encodes the MBS data 211 with the most robust of the available MCS levels. Although an MCS level specifies both a coding rate and a modulation scheme, only the coding rate is considered in FIG. 2. Therefore, encoding of the MBS data 211 is shown without modulation of the MBS data 211 in FIG. 2.
The BS creates a codeword 213 having the MBS data 211 and a parity by encoding the MBS data 211 with the most robust MCS level and generates a plurality of code symbols 215 out of the codeword 213. It is assumed herein that five code symbols 215-1 to 215-5 are produced from the MBS data 211. The BS sends the five code symbols 215-1 to 215-5 in a plurality of subframes, that is, five subframes 217-1, 217-3, 217-5, 217-7, and 217-9, respectively. A subframe can be an Orthogonal Frequency Division Multiplexing (OFDM) symbol, for example. Specifically, the BS sends the code symbol 215-1 in the subframe 217-1, the code symbol 215-3 in the subframe 217-3, the code symbol 215-5 in the subframe 217-5, the code symbol 215-7 in the subframe 217-7, and the code symbol 215-9 in the subframe 217-9.
MSs serviced by the BS receive the five subframes and recover the MBS data 211 by decoding the subframes. Since the MBS data should reach even the MS having the most inferior channel status, the BS sends the MBS data using the most robust MCS level. Hence, allocation of downlink resources corresponding to the most robust MCS level leads to the decrease of resource efficiency.
Other MSs in channel statuses other than the most inferior channel status, for example, MSs near the BS, also receive the MBS data using the most robust MCS level despite their good channel statuses. Thus, they suffer unnecessary processing power consumption during decoding of the MBS data. Specifically, the MSs receive all of the five OFDM symbols and Fast Fourier Transform (FFT)-process them to thereby recover the MBS data, resulting in unnecessary power consumption in the processing.